The present invention relates to a masking film mainly used in a photo-mechanical process or the like, wherein desired patterns are cut out from a light-shielding peelable layer of the film and unwanted portions are peeled off to form transparent images of the desired patterns.
As masking films used in the photo-mechanical process and the like, those having a light-shielding peelable layer provided on a transparent support are widely known. Such a light-shielding peelable layer requires various properties. Specifically, it should have good physical properties as a coated layer, good pick-up property (i.e., its edges should be easy to pick up from the transparent support), good peelability and anti-blocking property, and high stability of these properties with time.
A known masking film attempting to satisfy these requirements comprises a binder component of the light-shielding peelable layer consisting of a vinyl chloride resin such as vinyl chloride/vinyl acetate copolymers and vinyl chloride/vinyl acetate /maleic acid copolymers and a nitrile rubber having an acrylonitrile content of 19 to 51% by weight and a Mooney viscosity of 50 to 90.
However, this masking film does not provide all of the required properties. Further, it was found that, regardless of how the properties of the resin and the rubber were changed and/or how the weight ratio of the resin and the rubber was strictly controlled in such a masking film, the various required properties could not be obtained simultaneously. In other words, the above required properties cannot simultaneously obtained by using only one kind of the rubber with a vinyl chloride resin.
Under these circumstances, there has also been proposed a masking film comprising a binder component consisting of a vinyl chloride resin and a mixed nitrile rubber comprising one nitrile rubber having an acrylonitrile content of about 27% by weight and another nitrile rubber having an acrylonitrile content of about 33% by weight in a mixing ratio of 2:1 to 1:1 by weight (Japanese Patent Unexamined Publication KOKAI! No. 1-207753).
The use of the nitrile rubber having an acrylonitrile content of about 27% by weight in addition to one having a content of about 33% by weight does in fact increase adhesion of the light-shielding peelable layer. However, these nitrile rubbers in the mixing ratio mentioned above are likely to unduly increase the adhesion and hence markedly degrade the pick-up property and peelability of the light-shielding peelable layer. On the other hand, if the amount of the nitrile rubbers contained in the binder relative to the resin is decreased to reduce the adhesion of the peelable layer, the properties of the coated layers are likely to degrade with time.
Therefore, the object of the present invention is to provide masking films which can simultaneously satisfy the above-mentioned requirements, which have not been simultaneously satisfied by any conventional masking film, namely, appropriate flexibility, good pick-up property, peelability and anti-blocking property of the light-shielding peelable layer, and stability against degradation of these properties with time.